Operating temperatures of turbine systems are continuously being increased to provide increased efficiency. As the operating temperatures are increased, components of the turbine systems are modified to increase their temperature capability.
Common features of turbine system components include a variety of structures, base materials and surface treatments that are designed to provide cooling to a component of the system, such treatments including but not limited to, thermal, wear and corrosion barriers, cooling channels and microchannels on or near the surface of the component. There are benefits and disadvantages to all such features. In some particular examples, the cooling solutions are technically advantageous but are prohibitive due to cost and complexity, among other challenges.
A particular surface treatment of interest is layered coatings in the form of metallic foams or sponges, generically, porous coating structures. Examples of such porous coatings include foams made of aluminum. These are advantageous because they have very low specific weight and high compression strength combined with good energy absorption characteristics. The study of metallic foams has become attractive to researchers and engineers due to the range of potential applications for hot gas path articles such as turbines. Metallic foams are known and can be fabricated in three ways. According to one method, molten metals with adjusted viscosities are applied to an article or component of an article and are injected with gases or gas-releasing blowing agents which cause the formation of bubbles during their in-situ decomposition, thereby forming a porous coating. A second method involves the application to an article of supersaturated metal-gas systems under high pressure which initiates bubble formation whereby pressure and temperature control are employed to control formation of the foam to provide a porous coating. And a third method involves application of metal powders mixed with a blowing agent to the article and subjecting the mixture to heat treatment at temperatures near the melting point of the metal powder material, resulting in decomposition of the blowing agent and release of gas forcing the melting metal material to expand and forming a porous structure. Each of these known methods is costly and to the extent even in use, is typically suitable only for advanced technology components rather than broad use on turbine components.
There is a need in the art for alternatives to forming porous coating layers to provide cost effective thermal protection in turbine systems.